1. Field of the Invention
The present invention relates to a method for selectively exposing a peripheral portion of a substrate such as a wafer for integrated circuit manufacture, and an apparatus therefor.
2. Related Background Art
It is already known that photoresist, coated on a substrate such as a wafer for forming fine circuit patterns thereon, tends to be peeled off in the peripheral portion of the wafer, and thus peeled photoresist induces detrimental effects on the subsequent integrated circuit manufacturing process. For preventing such photoresist peeling in the peripheral portion of wafer, there is already known an apparatus disclosed in the Japanese Laid-open Patent Hei 2-56924 (corresponding to the U.S. patent application Ser. No. 396,557 filed Aug. 21, 1989). This conventional apparatus is provided with a light-emitting unit for emitting an exposing light beam and a light receiving unit for receiving said light beam, wherein said units are positioned mutually opposite the peripheral portion of the wafer and rendered integrally movable in the radial direction of the wafer. Within said light beam, a predetermined width (length in the radial direction of the wafer) is intercepted by the wafer, and the remaining light beam is received by the light receiving unit positioned opposite to the light emitting unit across the peripheral portion of the wafer, and the signal obtained in said light receiving unit is used for servo operation for maintaining a constant exposure width on the wafer. In such apparatus, therefore, if the intensity of the exposing light received by the light receiving unit for a given exposure width becomes significantly different from an anticipated level, for example because of a lowered intensity of the exposing light resulting from deterioration of the light source, or because of replacement of the light source, the correlation with the target value of the servo operation becomes unclear, so that the exposure width in the periphery exposing operation becomes unstable.
Also if the intensity of the exposing beam is not uniform in the radial direction but has, for example, a Gaussian distribution as shown in FIG. 6A, the signal from the light receiving unit is not linearly related with the exposure width as shown in FIG. 6B, so that it becomes difficult to control the exposure width solely by the light reception signal obtained at the exposure of the peripheral portion of the wafer.